this program factors a semiprime number.

semiprime_crack.cpp

#include <iostream>
#include <vector>//required of for the primes vector
#include <stdexcept>//out_of_range error handeling
#include <cmath>

/**semi-prime factoring
*Author: Dustin Van Tate Testa
*Purpose: To find the factors of a semi-prime
*Date: spring 2014
*/

//globals:
//experiment w/ long long ints
unsigned long int sub;//due to limitations, semi-prime must fit the data-type
std::vector<unsigned long int> primes;//holds al the primes
unsigned long int px = 0, py = 0;//primes place finders
bool isp, errcount = 0;//prevents infinite error messages
//functions:
void getprimes();//fill a vector (fancy arr.) of primes less than or equal to sub
void factr();

int main(){//interface(shit)
	std::cout <<"Give me a semi-prime to crack:";//prompt
	std::cin >>sub; //get semi-prime
	std::cout <<"\nGenerating Primes...\n"; getprimes(); px-=1;
	std::cout <<"\nFactoring:\n\n";
	factr();
	std::cout <<"\n";
	
}
//process functions
void factr(){//uses recursive process to find factors
	py = px--;
	isp = 0;
	for (unsigned int h = 0; h <= px; h++)
		for (unsigned int j = 0; j <= py; j++)
			if ((primes.at(j) * primes.at(h)) == sub) {//if j&h are factors of sub
				isp = 1;
				std::cout <<"-factors: (" <<primes.at(h) <<", " <<primes.at(j) <<")\n";//send answer
			}

	if (isp == 0)
		std::cout <<"The number has no prime factors\n\n";
}

void getprimes(){//fill a vector of primes less than or equal to sub
	unsigned long int i = 0, g;
	bool iprm = 1;
	while(i <= sub) {//count  up for i
		g = i - 1;
		iprm = 1;
		
		while (iprm && (g > 1)) //test i for primeness
			if (i % g-- != 0)
				iprm = 0;
		
		if (iprm) {
			//add i to the end of the vector (if i is prime)
			primes.resize(px); 
			primes.push_back(i);
			//cout <<"(i=" <<i <<")---(primes.at(" <<px <<")="<<primes.at(px) <<")---(size =" <<primes.size() <<")->" <<endl;//used only for testing
			px++;//note: must subtract one from final result
		}
		if (i++ >= 4) 
			i++;//save resources by incrementing two (evens aren't prime)(errors thrown before 4)
	}
}